Weed, insect resistance in cotton

Managing resistant insects and weeds goes well beyond important say Roger Leonard and Stanley Culpepper — it is essential from both economic and production standpoints to continue growing cotton in the South.

Leonard, an entomologist at LSU’s Ag Center, says one of the first glimpses of insect resistance seen by farmers in the Southeast was pyrethroid resistant budworm and bollworm. This was far from an unmanageable problem, but opened the eyes of entomologists as to future problems with advanced insecticide technology back in the 1970s.

“We didn’t know exactly what to make of the problem back then, but all of us in the cotton pest management business saw fields where pyrethroids didn’t work,” Leonard said, speaking at the recent Beltwide Cotton Conference in New Orleans.

When Bollgard came onto the market, the problem of pyrethroid-resistant insects highlighted the efficiency of these new products.

Before Bt cotton, growers in the Mid-South routinely sprayed an average of four to six times and up to 10 times for the tobacco budworm-bollworm complex. The early Bt products cut sprays by 50 percent. The new generation of Bollgard and WideStrike products will likely cut another 50 percent of insecticide use. This technology still won’t eliminate the need for insecticides in some fields, but it will significantly reduce the amount of conventional materials used by cotton farmers, Leonard says.

“Despite the success of these products in managing the bollworm-budworm complex, they do not make cotton insect-free. In fact, these materials accentuate the need for a well-trained professional to scout for secondary insects not controlled by Bt technology.”

Rotating insecticides is critical, Leonard stresses, to maintain a high level of efficacy and a relatively low cost.

Culpepper, a weed scientist at the University of Georgia, also speaking at the Beltwide meeting, says the reason for the dramatic increase in use of glyphosate-based weed management programs throughout the South is primarily simplicity of using these products, combined with there high level of efficacy in controlling a wide spectrum of weeds.

From 1997 to 2001 Roundup Ready cotton went from zero to 67 percent in the South. By 2006, at least 97 percent of the cotton in the Southeast was planted to Roundup Ready varieties.

In 2000, the first case of glyphosate resistant horseweed was confirmed, and this was further confirmed in cotton in 2001. Horseweed has not been a severe problem in cotton for the most part, but it did open the eyes of weed scientists across the country that there were changes occurring in weed management from glyphosate-based systems.

“Despite the growing evidence we would likely have future problems with glyphosate-resistant weeds, use of the technology continued to spread rapidly. Cotton acreage in the Mid-South and Southeast continued to increase well into the 2000s.

“In 2004, the first case of glyphosate resistance was documented in Palmer amaranth pigweed in Georgia. Growers who have faced this problem can attest that all weeds are not created equal, Culpepper says.

“By 2006, resistance was confirmed in five states and by 2009 glyphosate resistant pigweed was confirmed in 120 counties in eight states, and I’m soon to add another 10 to 12 counties in Georgia,” the weed scientist adds.

“We can no longer do as we did from 1999 to 2004 — rely primarily on glyphosate for control of Palmer amaranth — and a growing number of other weed species,” Culpepper says. “Those days are ending for many growers in the Southeast and they are ending very, very rapidly,” he adds.

“When a grower has multiple herbicide resistance in a weed species like Palmer pigweed, he’s got a real problem. When you lose two or more families of herbicides to resistance, for example ALS and glyphosate resistance, you to no longer have a reliable topical herbicide to control Palmer amaranth — that is a challenge.

“In Georgia, if you come to me with a weed problem, other than Palmer amaranth, I will have an answer for you — quickly. If you come to me with glyphosate resistant Palmer amaranth, I need to ask you a dozen or so questions before I can begin to give you a solution to your problem,” Culpepper says.

“Irrigation is a big question. If you have it, we have some options we don’t have on dryland cotton. If you have dryland cotton, we have some different options if you no-till versus using conventional-tillage. Palmer amaranth that cannot be controlled by glyphosate is just a challenging pest to manage.

“You can be the best farmer on the planet and you can go out and spend more than $50 per acre for weed control and still not be able to pick your cotton because you simply cannot manage resistant pigweed,” Culpepper stresses.

“My family farms, my two best friends in the world farm — I think farmers are the greatest people on earth. Still, my family, my friends and most of the farmers I know simply aren’t prepared for the impact Palmer amaranth has had and will have on many, many more acres of cotton in the South,” he adds.

“To win against this pest we are going to have to develop integrated weed management strategies that we didn’t we even dream about back when we were openly embracing this new Roundup Ready technology.”

“For example, a grower can go out and spend the $50-plus in herbicides and unless he or she can make it rain, they are very likely to lose against Palmer amaranth. If the grower integrates a deep tillage program and turns enough weed seed under ground — and it appears at 18 inches below the surface Palmer amaranth is killed off in large numbers — the $50 herbicide investment has a much better chance of working.

“For conservation-tillage growers, you can get the same kind of benefit from planting a rye cover crop to reduce the number of Palmer amaranth seed that emerge. These are the types of things growers don’t want to do, but must do to be competitive with Palmer amaranth.

“Deep tillage and a rye cover crop together work better than either works alone, but too few farmers either don’t have the equipment to do both or are not willing to switch from one tillage system to another.

“To be successful, these integrated programs still must include herbicides. Some ask, “why use a herbicide, if I only get a low percentage of weeds controlled. The answer is simple, without herbicides, especially residual herbicides in the system, there is no chance for success,” Culpepper explains.

“In a dryland, conservation-tillage system with glyphosate resistance, an Ignite-based herbicide program will work better than a glyphosate-based program. Ignite-based systems are highly time sensitive — you just don’t have the same window of opportunity for killing pigweed with Ignite that you had with glyphosate. Growers, in many cases, have to learn how to use Ignite — it is a different herbicide than glyphosate,” Culpepper says.

Leonard and Culpepper agree the tools are there to manage insect and weed resistance problems, but it will take some different thinking to put these tools to best use in different crop production systems.